How do u decorate or change glass block on kitchen island?

Please send a picture. True glass block can be chemically etched and one can go to a stain glass supply store & buy that chemical.

If you just want to decorate it, like for the holidays, you can attach some strands of the mini christmas lights. The light flickering off the glass would look beautiful! If you're looking to permanently change the look of the glass without removing it you can paint them w/glass paint.

You could paint them to give them a stained glass look.

They have stick on decals that look like tile 5here nice ebay